Cans, such as beverage cans, food cans, 18-liter cans, and pail cans, are roughly classified into two-piece cans and three-piece cans, based on their manufacturing method (process).
In the two-piece can, a can bottom and a can body are integrally formed by, for example, a shallow drawing process, a drawing and wall ironing process (DWI process), or a drawing and redrawing process (DRD process) of a surface-treated steel sheet, which is provided with treatment such as tin plating, chromium plating, metal oxide coating, chemical conversion coating, inorganic film coating, organic resin film coating, or oil coating. Then, this is provided with a lid to give a can consisting of two parts.
In the three-piece can, a can body is formed by bending a surface-treated steel sheet into a round tube or a rectangular tube and jointing the ends thereof. Then, this is provided with a top lid and a bottom lid to give a can consisting of three parts.
In these cans, the ratio of material costs to can costs is relatively high. Therefore, to reduce the can costs, it is strongly required to reduce the costs of steel sheets. In particular, due to the recent steep rise in steel sheet prices, in the can manufacturing field, it has been tried to reduce material costs by using a steel sheet thinner than conventional ones. On this occasion, there is a demand for steel sheets having high strength to compensate for a decrease in can strength due to a decrease in the thickness.
For example, when an ultrathin steel sheet having an thickness of 0.14 to 0.15 mm is used, to ensure sufficient pressure capacity of the can body and the top and bottom lids of a three-piece can or the can bottom of a two-piece can, a strength of at least about 600 to 850 MPa in terms of tensile strength (TS) is necessary.
The presently existing ultrathin steel sheets for cans having high strength are manufactured by a double reduce method (hereinafter referred to as DR method) in which secondary cold rolling is performed after annealing. The strength of steel sheets mainly manufactured by the DR method is a level of 550 to 620 MPa in terms of TS. That is, the DR method is practically used for those having a strength level slightly lower than the strength of 600 to 850 MPa that is required in the above-mentioned steel sheets having thicknesses of about 0.14 to 0.15 mm. This is based on the following reasons.
That is, since the DR method strengthens a steel sheet by work hardening through secondary cold rolling, the organizational characteristics of the steel shows a high dislocation density. Therefore, the ductility is low. In a material having a strength of about 550 MPa, the total elongation (El) is about 4% or less, and in a material having a strength of about 620 MPa, it is about 2% or less. In some manufacturing examples, the steel sheet has a strength of about 700 MPa, but is very poor in ductility, such as an El of about 1% or less. Therefore, the steel sheet is used only in limited application that does not require machining thereof. That is, the steel sheet is not applied to a main use of steel sheets for cans, such as can bodies, top lids, and bottom lids of three-piece cans or two-piece cans.
In addition, as described above, in the DR method, steel sheets are manufactured through a process including hot rolling, cold rolling, annealing, and secondary cold rolling. That is, the process includes a larger number of steps than the common method that is completed at the step of annealing, and, therefore, the manufacturing cost thereof is high. Thus, the steel sheets obtained by the DR method not only have insufficient strength but also are inferior in ductility and high in manufacturing cost.
Accordingly, methods for solving these disadvantages of the conventional DR materials have been investigated.
For example, Japanese Unexamined Patent Application Publication No. 4-280926 discloses a method of manufacturing a steel sheet for cans, wherein Nb, which is an element forming a carbonitride, is added to an ultra-low carbon steel; hot rolling is performed at a temperature not higher than the Ar3 transformation point (also referred to as Ar3 point), namely, in an a region; and annealing is not performed after the cold rolling. However, the steel sheet obtained by the technique of JP '926 is in the state after that the cold rolling has been conducted and is therefore poor in ductility and does not have sufficient workability for some purposes.
As a technique for improving these problems, Japanese Unexamined Patent Application Publication No. 8-41549 discloses a technique for improving ductility by adding Nb and Ti, which are elements forming carbonitrides, to an ultra-low carbon steel and performing hot rolling at a temperature not higher than the Ar3 point, cold rolling, and then low-temperature annealing. The term “low-temperature annealing” used herein is annealing that is performed at a temperature not to cause recrystallization, and, therefore, the energy cost for heating is reduced.
In addition, Japanese Unexamined Patent Application Publication No. 6-248339 discloses a technique involving adding Nb, Ti, Zr, V, and B, which are elements forming carbonitrides, to an ultra-low carbon steel and performing hot rolling at a temperature not higher than the Ar3 point, cold rolling, and then annealing at a temperature not higher than the recrystallization temperature.
The characteristics common in JP '926, JP '549 and JP '339 are that an ultra-low carbon steel is used as the steel; elements forming carbonitrides are added; and the hot rolling is performed at a temperature not higher than the Ar3 point. However, the steel sheets manufactured under these conditions have a problem of insufficient uniformity in thickness in the longitudinal direction of the steel sheet coil.
In JP '549 and JP '339, steel sheets having high strength are obtained by performing annealing not involving recrystallization. In the hot rolling performed in these technologies, rolling of 40% or 50% or more is performed at a temperature not higher than the Ar3 point. In such a case, even if the annealing does not involve recrystallization, a TS of 600 to 850 MPa, which is desirable, cannot be obtained.
It could therefore be helpful to provide a method of manufacturing a steel sheet for cans having high strength and ductility necessary for a canning process, while inhibiting the variation in thickness in the longitudinal direction of the steel sheet coil.